This may show more information on your specific problem:

https://www.google.com/safebrowsing/diagnostic?site=YourDomain.com

I've had this happen once before on a site the problem was the Guru Inc infection although that was back in 2015 and store was barely maintained. 

See here about that: https://support.hypernode.com/knowledgebase/how-to-fix-guruinc-infection/ 

The malware should be removed and then once this is complete you can submit to google for removal of the warning with Google Webmasters.